1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include a network of base stations, base station routers, and/or other wireless access points that are used to provide wireless connectivity to access terminals in geographic areas (or cells) associated with the network. Information may be communicated between the network and the access terminals over an air interface using wireless communication links that typically include multiple channels. The channels include forward link (or downlink) channels that carry signals from the base stations to the access terminals and reverse link (or uplink) channels that carry signals from the access terminals to the base station. The channels may be defined using time slots, frequencies, scrambling codes or sequences, or any combination thereof. For example, the channels in a Code Division Multiple Access (CDMA) system are defined by modulating signals transmitted on the channels using orthogonal codes or sequences. For another example, the channels in an Orthogonal Frequency Division Multiplexing (OFDM) system are defined using a set of orthogonal frequencies known as tones or subcarriers.
Next (4th) generation wireless systems such as 802.16e WiMAX, UMTS Long Term Evolution (LTE) and cdma2000 EV-DO Revision C Ultra Mobile Broadband (UMB) are based on Orthogonal Frequency Division Multiplexing (OFDM). In OFDM, the transmitted signal consists of narrowband tones (or sub-carriers) that are nearly orthogonal to each other in the frequency domain. A group of tones transmitted over the duration of one time slot (or frame) constitutes the smallest scheduling resource unit, also known as a tile, a resource block (RB), or a base node (BN). Different tones belonging to a tile may be scattered across the entire carrier frequency band used by the OFDM system so that each tile transmission experiences diversified channels and interference on each sub-carrier. Alternatively, a tile can be formed of a contiguous set of tones so that the channel and interference experienced by the tile are more localized.
Conventional wireless systems are based on a static allocation of the spectrum used for wireless communications. For example, a first portion of the spectrum can be pre-allocated or licensed to a first communication system to support wireless communication with access terminals that are registered with the first communication system. A second portion of the spectrum could then be allocated to a second communication system. Alternatively, the second portion of the spectrum could be used for unlicensed communications over the air interface. If the wireless system supports OFDM communication then the first and second portions of the spectrum may include tones, subcarriers, and/or sub-bands of the total bandwidth allocated for OFDM communication. Alternatively, if the wireless system supports other types of communication that allocate the bandwidth of the available spectrum in different ways, e.g., using different sets of frequencies, then the spectrum may be partitioned into first and/or second portions in other ways.
Static allocation of the available spectrum is used in conventional wireless communication at least in part because it is comparatively simple to implement. However, static allocation may lead to inefficient use of the spectrum resources. For example, the spectrum allocated to the first communication system for unlicensed communications may be underutilized, e.g., due to a small number of subscribers. At the same time, the spectrum allocated to the second communication system and/or unlicensed communications may be over utilized if a large number of access terminals are attempting to access the second communication system and/or use the unlicensed communication bands. This may lead to lowered quality of service, dropped calls, denial of access requests, and other undesirable effects.
Spectrum flexibility is therefore expected to become an important characteristic of future wireless communication systems. Spectrum flexibility is achieved in an OFDM system by dynamically nulling out sub-carriers in the bands occupied by other communication systems. For example, different OFDM communication systems may utilize different sub-bands depending on the current or expected resource requirements for each system. Future OFDM-based communication systems are also likely to employ a noncontiguous OFDM transmission technique in which the sub-carriers that are actually used by the system are not necessarily contiguous and could be scattered over a large bandwidth for spectrum flexibility. Such a scheme retains the spectral efficiency advantages of OFDM by minimizing guard-bands while providing the possibility for dynamic spectrum occupancy.
Two fundamental problems need to be addressed to implement spectrum flexibility in OFDM systems. First, the communication system has to provide a way for the receiver to synchronize to the symbols transmitted over the air interface, i.e., by identifying the sample that corresponds to the start of an OFDM symbol. Second, the communication system has to tell the receiver which sub-bands are actually being used by the system for communication over the air interface. One conventional solution is to implement a separate signaling channel that carries signals that indicate the bandwidth over which the traffic signal is transmitted. This solution requires that a portion of the bandwidth be reserved for the control signal and the receiver needs to monitor this out-of-band control signal continually. Current OFDM symbol synchronization techniques rely on the use of a cyclic prefix or a time-domain preamble signal that is transmitted over a contiguous band that is a priori known to the receiver. Thus, conventional symbol synchronization techniques are not applicable to noncontiguous OFDM signaling.